1. Field of the Invention
The present invention relates generally to unsymmetrical squaraine dyes which exhibit nonlinear optical (NLO) properties. More particularly, the present invention relates to materials which have first molecular electronic hyperpolarizability (.beta.) and therefore display second order nonlinear optical properties.
2. Description of Related Art
Organic materials that show second-order nonlinear optical responses are of interest for a variety of photonic and opto-electronic applications. See Marder, S. R., Sohn, J. E. & Stucky, G. D. eds. Materials for Nonlinear Optics: Chemical Perspectives, ACS Symposium Series, Vol. 455(American Chemical Society, Washington, 1991); Chemla, D. S. & Zyss, J. eds Nonlinear optical properties of Organic Molecules and Crystals, Vol. 1 and 2 (Academic Press, San Diego, 1987); and Williams, D. J. Agnew. Chem. Int. Ed. Engl. 23, 690-703 (1984).
Exemplary nonlinear optical materials and devices which utilize such materials are described in U.S. Pat. Nos. 5,062,693; 5,011,907; and 5,016,063. Nonlinear optical materials are also described in Japanese Patent Appln. No. 63-270834 filed Oct. 28, 1988, and published on May 2, 1990. Squaric acid derivatives which have been investigated for second harmonic generation include 1,2-disubstituted cyclobutene-3,4-diones. Exemplary materials have been described in Japanese Patent Appln. Nos. 89,204,031; 90,259,735; 91,71,117; 91,112,961; 92,199,135; 92,202,165; 92,202,166; and 92,202,167.
There is a continuing need to develop new materials which have sufficiently high second-order nonlinear optical properties when used in thin films and crystals to make them useful for applications such as telecommunications, optical data storage and optical information processing.